In conventional lamp, when a reflector cover is assembled, the reflector cover is generally mounted to a lamp holder by a threaded connection. There is a rolling operation in the locking process. However, in the process of the rolling operation, in order to prevent from being driven synchronously, the lamp holder or the reflector cover is required to be held, thus it is inconvenient for assemble or dismantle operation, and a risk of rupture exists due to different locking power of individuals.